My guess is there's something wrong with an entity file that changes its mesh after research or an upgrade.
that would be my guess as well, or a particle that uses a mesh. You need to test more as you go. If you add a mesh or a mesh to an upgrade or particle, test ,test ,test.
Now you are left with playing same race on all teams and see which race the error belongs to as the AI player may cause the error and you will not see it. Then once you find which race, keep test playing until you find it while playing, like upgrading and when the research finishes it errors.
Good luck, I have been there.
Crap, team login